1. Field of the Invention
The present invention relates to an image processing apparatus for processing an image, a data processing apparatus and a method therefor.
2. Related Background Art
For reversible encoding of a multi-value image, there is already known DPCM (differential pulse code modulation). In general, the encoding by DPCM is achieved by calculating the predicted value (plane prediction) of an encoding target pixel, utilizing already encoded pixels around the encoding target pixel, for example a pixel (value a) at the left side, a pixel (value b) at the upper side and a pixel (value c) at the upper left position, for example according to a calculation formula (a+b-c), and encoding the difference between thus calculated predicted value and the value of the encoding target pixel.
The multi-value gradation image, represented by image data obtained scanning a natural image such as a photograph, show significant difference in the statistical property depending on the position within the image. Such statistical difference arises from the difference in the local image structure, such as the presence of an edge in the image or the direction of inclination of the luminance.
The above-mentioned prediction of the pixel value according to the DPCM method is only effective, among such local structures, for those that can be approximated partially to a two-dimensional plane, and is detrimental to other structures because of an increase in the dynamic range.
For complementing such encoding method, there can be conceived an encoding method of effecting the encoding in the unit of a block, employing an encoding method effective for the structure prevailing in the block and attaching an index, indicating such encoding method, as additional information.
However the local structure within the image generally occupies a large area in a flat (low frequency structure) portion but becomes as small as several pixels in an edge portion or a fine line portion with steep change in the luminance.
Consequently, in case of encoding in the unit of a block, a larger block size is more efficient in such flat portion and a smaller block size is more efficient in the fine structure portion.
However, even such flat portion may have a complex shape, and even the high frequency structure may extend uniformly over a wire area. Conventionally it has not been possible to effect encoding with the optimum encoding method, utilizing the optimum encoding unit according to the property of the image as explained above.
Also in case of switching the encoding method in the unit of a block, there is conventionally conducted the Huffman encoding, utilizing a predetermined Huffman table for all the blocks. Consequently the efficiency of encoding is undesirably low, as the encoding with optimum entropy cannot be executed for each of the encoding methods used for such encoding block units.